You shouldn't need separate projects for 32-bit and 64-bit. And if you are writing solely a .NET program, with no Interops, PInvokes, or such stuff, you shouldn't need separate builds for the different variants of Autocad. Just reference the managed DLLs from the ObjectARX 2010 directory, and make sure you turn off the "Copy Local" option.
You can run into problems on your target machines, however, if you have side-by-side installs of 32-bit and 64-bit variants of Autocad from the same year. The problem is that the assemblies get installed into something in Windows called the Global Access Cache, or GAC. For any given release of Autocad, both the 32-bit and 64-bit versions of AutoCAD register their assemblies using the same GUID. So this value can only point to either 32-bit Autocad or 64-bit Autocad. The upshot is that your .NET assemblies will only run on one of them. For example, it might work fine on your 32-bit installations of Autocad products, but you get an E_NOINTERFACE error on your 64-bit installs. Or vice-versa.